How to create properties for new object types
bpy.types.Object.povcone = PointerProperty(type=RenderPovSettingsCone_add)
bpy.utils.register_class(bpy.types.Object.povcone)


you do not need to register a property from bpy.props like this 

the property is there after the assignment and blender will use it as a wrapper for 

obj.["povcone"]


so simply drop the second line